Saturday, October 25, 2008

Cross Staff.....

The cross-staff in use in 1579 was a simple device that worked reasonably well for measuring the angle of the sun above the horizon at noon. It was fitted with one movable vane (transversary) that, with the end of the staff placed at the eye of the observer, was positioned so that it appeared to touch both the horizon and the sun. The angle was then read from a scale on the staff. In his Regiment, Bourne admonishes the navigator against using the cross-staff for measuring an altitude of over 50°, the maximum angle that the sun and the horizon could be taken in together with one glance. The cross-staff was the method of resort on a rocking ship since its use did not rely on gravity.

The quadrant is a very simple instrument of medieval origin used to determine the altitude of a heavenly body. It takes it name from its shape, which is a quarter of a circle. The curved edge is divided from 0 to 90 degrees. At the apex is a right angle, where a cord with a small weight, or plumb-bob, of lead or brass is attached. Along one straight edge are mounted two upright pieces with holes for sighting. When in use, the quadrant is held vertically so the plumb-line falls across the scale of degree markings, and from this the angle of elevation can be read.